Stem cells have a capacity both for self-renewal and the generation of differentiated cell types. This pluripotentiality makes stem cells unique. In addition to studying the important normal function of stem cells in the regeneration of tissues, researchers have further sought to exploit the potential of in situ and/or exogenous stem cells for the treatment of a variety of disorders. While early, embryonic stem cells have generated considerable interest, the stem cells resident in adult tissues may also provide an important source of regenerative capacity.
These somatic, or adult, stem cells are undifferentiated cells that reside in differentiated tissues, and have the properties of self-renewal and generation of differentiated cell types. The differentiated cell types may include all or some of the specialized cells in the tissue. For example, hematopoietic stem cells give rise to all hematopoietic lineages, but do not seem to give rise to stromal and other cells found in the bone marrow. Sources of somatic stem cells include bone marrow, blood, the cornea and the retina of the eye, brain, skeletal muscle, dental pulp, liver, skin, the lining of the gastrointestinal tract, and pancreas. Adult stem cells are usually quite sparse. Often they are difficult to identify, isolate, and purify. Often, somatic stem cells are quiescient until stimulated by the appropriate growth signals.
Progenitor or precursor cells are similar to stem cells, but are usually considered to be distinct by virtue of lacking the capacity for self-renewal. Researchers often distinguish precursor/progenitor cells from stem cells in the following way: when a stem cell divides, one of the two new cells is often a stem cell capable of replicating itself again. In contrast, when a progenitor/precursor cell divides, it forms two specialized cells, neither of which is capable of replicating itself. Progenitor/precursor cells can replace cells that are damaged or dead, thus maintaining the integrity and functions of a tissue such as liver or brain.
Muscle tissue in adult vertebrates regenerates from reserve myoblasts called satellite cells. Satellite cells are distributed throughout muscle tissue and are mitotically quiescent in the absence of injury or disease. Following recovery from damage due to injury or disease or in response to stimuli for growth or hypertrophy, satellite cells reenter the cell cycle, proliferate and undergo differentiation into multinucleate myotubes, which form new muscle fiber. The myoblasts ultimately yield replacement muscle fibers or fuse into existing muscle fibers, thereby increasing fiber girth by the synthesis of contractile apparatus components. This process is illustrated, for example, by the nearly complete regeneration that occurs in mammals following induced muscle fiber degeneration or injury; the muscle progenitor cells proliferate and fuse together to regenerate muscle fibers.
Vertebrate muscles are thought to originate in the embryo from mesoderm-derived cells of the dorsal somites. During muscle development, some somite-derived myogenic progenitors do not differentiate into myofibers and instead are retained as muscle stem cells, or satellite cells, located beneath the basal lamina of muscle fibers. Satellite cells first appear in the limb muscles of mouse embryos between 16 and 18 days post conception (dpc). In neonatal mice, satellite cell nuclei comprise ˜30% of myofiber-associated nuclei, but their number declines with age and only ˜5% of myofiber nuclei in the muscles of adult mice represent satellite cells.
In injured adult muscle, satellite cell number and regenerative capacity remain nearly constant through multiple cycles of regeneration, suggesting that these cells may be capable of self-renewal, or that this population is maintained by self-renewing satellite cell precursors. Currently, satellite cells are defined both positionally, by their location beneath the basal lamina, and functionally, by their ability to undergo myogenic differentiation; however, potential heterogeneity in the function and/or origin of sublaminar myogenic cells may exist and has yet to be fully addressed.
In recent years, reports of adult skeletal muscle progenitors distinct from satellite cells have accumulated. For example, muscle-resident side population (muSP) cells, defined by their ability to exclude Hoechst 33342 and representing a population distinct from satellite cells, have been shown to contribute to myofibers when injected intramuscularly (McKinney-Freeman et al., 2002) or when co-cultured with myoblasts (Asakura et al. (2002) J Cell Biol 159,123-34), although muSP cells appear to lack myogenic activity when cultured alone.
Likewise, muscle-resident CD45+Sca-1+ cells fail to generate myogenic cells in vitro when cultured alone, but acquire myogenic potential when co-cultured with primary myoblasts or in response to muscle injury or activation of Wnt signaling by LiCl (Polesskaya et al. (2003) Cell 113, 841-52).
In addition, cells with high proliferative potential and the ability to differentiate into multiple cell types, including muscle, neural, endothelial, and hematopoietic lineages, have been isolated from muscle (Cao et al. (2003) Nat Cell Biol 5, 640-6; Qu-Petersen et al. (2002) J Cell Biol 157, 851-64). Finally, bone marrow cells recently have been suggested to contribute to myofibers when injected directly into injured muscle or intravenously into injured (Fukada et al. (2002) J Cell Sci 115, 1285-93) or mdx dystrophic animals (Ferrari et al. (2001) Nature 411, 1014-5). Even single hematopoietic stem cells (HSC), which reconstitute the entire hematopoietic system (Wagers et al. (2002) Science 297, 2256-9), also contribute at a low-level to skeletal myofibers following severe muscle injury (Camargo et al. (2003) Nat Med 9, 1520-7; Corbel et al. (2003) Nat Med 9, 1528-32).
However, whether contributions of BM cells to injured skeletal muscle proceed through the generation of muscle-resident satellite cell intermediates remains controversial. While some studies have reported the derivation of muscle-resident satellite cells from transplanted BM cells (LaBarge and Blau (2002) Cell 111, 589-601), others have suggested that donor-marker expressing myofibers arise via fusion of donor hematopoietic cells into existing host myofibers.
The ability to manipulate muscle regeneration is of great interest for clinical and research purposes. Characterization of stem and progenitor cells having myogenic potential is therefore of great interest.